(57) A discrete event simulation system that avoids all blocking and advances the simulation time in an efficient manner by treating the simulated system as a set of subsystems and simulating the subsystems concurrently. The simulation proceeds iteratively by restricting the simulation of scheduled events for each subsystem at any one time to a chosen simulated time segment (bounded lag) beginning with the lowest simulation time found among the subsytems. With each simulation iteration, an "at risk" demarcation time is evaluated based only on a subset of the subsystems that can potentially affect the simulation at the considered subsytem. Events scheduled for a time earlier than the "at risk" time are simulated. In simulating systems where some subsystems affect other subsystems only through intermediate subsystems, opaque periods can be experienced when, because of the specific process that is being simulated, such an intermediate subsystem "promises" that a particular route maintaining from this subsystem would be busy for a set period of time, and thereby also "promises" that no other subsystem can use this route as a conduit to affect other subsytems. That tends to push forward the "at risk" demarcation time.